Bound-free pair production from nuclear collisions and the steady-state quench limit of the main dipole magnets of the CERN Large Hadron Collider
Michaela Schaumann, John M. Jowett, Cristina Bahamonde Castro, Roderik Bruce, Anton Lechner, Tom Mertens
BBound-free pair production from nuclear collisionsand the steady-state quench limit of the main dipole magnetsof the CERN Large Hadron Collider
M. Schaumann, ∗ J.M. Jowett, C. Bahamonde Castro, R. Bruce, A. Lechner, and T. Mertens
CERN, Geneva, Switzerland (Dated: August 13, 2020)During its Run 2 (2015–2018), the Large Hadron Collider (LHC) operated at almost twice higherenergy, and provided Pb-Pb collisions with an order of magnitude higher luminosity, than in theprevious Run 1. In consequence, the power of the secondary beams emitted from the interactionpoints by the bound-free pair production (BFPP) process increased by a factor ∼
20, while thepropensity of the bending magnets to quench increased with the higher magnetic field. This beampower is about 35 times greater than that contained in the luminosity debris from hadronic in-teractions and is focused on specific locations that fall naturally inside superconducting magnets.The risk of quenching these magnets has long been recognized as severe and there are operationallimitations due to the dynamic heat load that must be evacuated by the cryogenic system.High-luminosity operation was nevertheless possible thanks to orbit bumps that were introducedin the dispersion suppressors around the ATLAS and CMS experiments to prevent quenches bydisplacing and spreading out these beam losses. Further, in 2015, the BFPP beams were manipulatedto induce a controlled quench, thus providing the first direct measurement of the steady-state quenchlevel of an LHC dipole magnet. The same experiment demonstrated the need for new collimatorsthat are being installed around the ALICE experiment to intercept the secondary beams in thefuture. This paper discusses the experience with BFPP at luminosities very close to the future HighLuminosity LHC (HL-LHC) target, gives results on the risk reduction by orbit bumps and presentsa detailed analysis of the controlled quench experiment.
I. INTRODUCTION
In its second major physics program, the Large HadronCollider (LHC) [1] operates with nuclear beams to studystrongly-interacting matter—notably the Quark-GluonPlasma—at the highest temperatures and densities avail-able. For about one month at the end of each operationalyear, the LHC collides fully stripped lead ( Pb ) ionswith each other or with protons. So far four full Pb-Pbruns have been executed in the years 2010, 2011, 2015and 2018 [2–7] . The LHC has four interaction points(IPs) that host the main experiments ATLAS (IP1), AL-ICE (IP2), CMS (IP5) and LHCb (IP8). Since 2015 allof them have been participating in Pb-Pb data taking .Details of the operational conditions and differences be-tween the interaction regions (IRs), as well as achieved lu-minosities will be given in Section II. Table I summarisesthe Pb beam parameters from the original LHC design,the maximum achieved in operation and those expectedfor high-luminosity operation in Run 3 (starting in 2022).Major operational challenges and luminosity limits inPb-Pb operation of the LHC originate from those in-teractions between lead nuclei in the colliding buncheswhich have impact parameter b > R , where R is thenuclear radius. Since the nuclei do not overlap these ∗ [email protected] Additionally three p-Pb runs took place in 2012, 2013 and2016 [8, 9]. LHCb was the last experiment to join the heavy-ion community,taking its first ion collisions in the pilot p-Pb run in 2012. ultra-peripheral interactions are purely electromagnetic.Among many possible reactions, two effects dominate:(1) copious lepton-pair production in collisions betweenquasi-real photons, and (2) emission of nucleons in elec-tromagnetic dissociation (EMD) of the nuclei, dominatedby excitation of the Giant Dipole Resonance. Most ofthe pair production is innocuous except for the (single)bound-free pair production (BFPP1): Pb + Pb −→ Pb + Pb + e + , in which the electron is created in a bound state of onenucleus. Among the EMD processes, the channels whereone nucleus loses either one or two neutrons are the mostfrequent: Pb + Pb −→ Pb + Pb + n , Pb + Pb −→ Pb + Pb + 2n . As extensively discussed previously (see, e.g., [4, 11–15] and further references therein), the modified nucleiemerge from the interaction point (IP), as a narrow sec-ondary beam with modified magnetic rigidity, following adispersive trajectory. This is illustrated in Fig. 1, wherethe red line indicates the trajectory of the BFPP1 ionsthat separate from the main beam (blue) when they en-ter the dispersion suppressor (DS) downstream from theIP. This beam impacts over just a few meters longitudi-nally, on the beam screen in a superconducting magnetof the DS, giving rise to a localized power deposition inthe magnet coils. These secondary beams emerge in bothdirections from every IP where ions collide. Each carriesa power of P p = Lσ p E b , (1) a r X i v : . [ phy s i c s . acc - ph ] A ug LHC design 75 ns (2018) HL-LHC
Beam energy [Z TeV] 7 6.37 7Total number of bunches 592 733 1240Bunch intensity [10 Pb ions] 7 21 18Normalized transverse emittance [ µ m] 1.5 2.3 1.65RMS bunch length [cm] 7.94 8.24 7.94 β ∗ in IP (1/2/5/8) [m] (0.55 / 0.5 / 0.55 / 10.0) (0.5 / 0.5 / 0.5 / 1.5) (0.5 / 0.5 / 0.5 / 1.5)Net crossing-angle IP (1/2/5/8) [ µ rad] (160 / 40 / 160 / -) (160 / 60 / 160 / 320) (170 / 100 / 170 / 305)Peak luminosity IP (1/2/5/8) [ cm − s − ] (1.0 / 1.0 / 1.0 / -) (6.1 / - / 6.1 / -) -Levelled luminosity IP (1/2/5/8) [ cm − s − ] - (- / 1.0 / - / 1.0) (7.0 / 7.0 / 7.0 / 1.0)TABLE I. Pb beam and main optics parameters at collision in the LHC design report [1], as achieved in 2018 [6, 7], and asenvisaged for HL-LHC [10]. The 2018 parameters refer to the average typical in the fills with 75 ns bunch spacing. x [ m ] M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B L E G R . R . B M B . A R . B M B . B R . B M B . C R . B Beam 1 Pb (main beam) Pb (BFPP1) FIG. 1. Example of main Pb (blue, 10–12 σ ) andBFPP1 Pb beam (red, 1–2 σ ) envelopes, and aperture(grey) in the horizontal plane, Beam 1 direction right of IP5(at s = 0). Beam-line elements are indicated schematicallyas rectangles. Dipoles in light blue, quadrupoles in dark blue(focusing) and red (defocusing). While the main beam travelsthrough the center of the beam-line elements in the disper-sion suppressor (starting at about 250 m), the BFPP1 beamseparates and impacts in the aperture of the second super-conducting dipole magnet of cell 11. where L is the instantaneous luminosity, σ p the interac-tion cross-section of the corresponding process (BFPP1,EMD1, EMD2, etc.) and E b is the beam energy. Atthe 2015/18 beam energy of E b = 6 . Z TeV [5, 7],the theoretical cross-section for the BFPP1 process is σ BFPP1 (cid:39)
276 b [16]. Cross-sections for the EMD pro-cesses are σ EMD1 (cid:39)
95 b and σ EMD2 (cid:39)
30 b [17, 18]. AsEq. (1) shows, these losses carry much greater power thanthe luminosity debris from nuclear interactions of totalcross-section 8 b.Since these effects are directly proportional to theluminosity, they will be of even greater concern afterthe high-luminosity upgrade that is being implementedin the current Long Shutdown (LS2, 2019-2021). Thekey upgrades that will have influence on the secondary Measurements are not available at the time of writing. beam power are the lifting of the limit on peak lumi-nosity in the ALICE experiment from the current L =1 × cm − s − to about L = 7 × cm − s − [19],and the RF upgrade in the Super Proton Synchrotron(SPS) that will reduce the bunch spacing to 50 ns [20, 21]allowing for a higher circulating beam intensity and in-creased potential peak luminosity in all experiments.From a comparison of the interaction cross-sections, itis clear that the BFPP1 secondary beam poses the great-est risk. It carries enough power to quench magnets anddirectly limit luminosity (see e.g. [22, 23]). The quenchexperiment that will be discussed in detail in Section IIIshowed that the BFPP beams can quench a supercon-ducting dipole at a luminosity of L ≈ . × cm − s − ,if the full secondary beam impacts directly in the mag-net. The EMD beams are of less concern, because theirpower is about 2.9 times lower than that of the BFPP1beam. Moreover, the rigidity change of the EMD1 issmall enough such that those particles do not impact inthe DS, but continue travelling on their dispersive trajec-tory until they are intercepted by the momentum clean-ing collimators. For those reasons, the discussions in thispaper will concentrate on the BFPP beams and theirconsequences.The phenomena discussed in this paper are only signif-icant for the Pb-Pb colliding beam mode. The produc-tion of secondary beams in p-Pb collisions is negligible,because of the much reduced interaction cross-section.The BFPP cross-section in p-Pb collisions is only around40 mb [24] although the corresponding luminosity is twoorders of magnitude greater. In high luminosity proton-proton collisions, the cross-section, at a few pb, is muchsmaller still and results in an occasional ( ∼ . II. OPERATIONAL EXPERIENCE IN RUN 2A. General Machine Configuration
In this paper, we focus on the two Pb-Pb operation pe-riods in Run 2. In each of them, the IPs of ATLAS, AL-ICE and CMS were operated with identical β -functions:
400 200 0 200 400s [m] from IP50.040.020.000.020.04 x [ m ] M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B L E G R . R . B M B . A R . B M B . B R . B M B . C R . B M B . A L . B M B . B L . B M B . A L . B M B . B L . B M B . A L . B M B . B L . B M B . A L . B M B . B L . B L E F L . L . B M B . A L . B M B . B L . B M B . C L . B Beam 2 Beam 1
IP5 Pb
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FIG. 2. Full view of IR5 and adjusted DS. The incoming beam envelopes, before the collision at IP5, are not shown. The greyshaded area represents the aperture, the coloured rectangles on the top the beam-line elements. The effect of a − s = ±
440 m) on the main ( Pb ) and BFPP ( Pb ) 1 σ beam envelopes is shown. Note that theorigin of all beams lies at IP5 (center of the plot) such that Beam 2 travels to the left and Beam 1 to the right.
410 420 430 440 450s [m] from IP50.000.010.020.030.04 x [ m ] M B . B R . B L E G R . R . B M B . A R . B Beam 1 Pb (BFPP1) - no bump Pb (BFPP1) - 3 mm bump FIG. 3. Zoom into Fig. 2 at the impact location of the BFPPbeam right of IP5. Purple trajectory calculated without orbitbump, red with a bump amplitude of − β ∗ = 0 . β ∗ = 0 . L = 1 × cm − s − [1].ATLAS and CMS do not have such a limit and couldaccept the maximum available peak luminosity. There-fore, and thanks to the injector performance well be-yond design, L = 3–3.5 × cm − s − was achieved in2015 [5]. This record was broken again in 2018 [7], whenthe reduced β -functions and a further improvement inthe injector performance [26–29], including shorter bunch spacing and higher single bunch intensities, led to a peakluminosity of L = 6.1 × cm − s − in these experi-ments. Thus the BFPP1 beams emerging from the leftand right of the ATLAS and CMS experiments were car-rying a power of up to P BFPP ≈
140 W, which is, as weshall show later, enough to provoke a quench.For LHCb, 2015 was the first year of Pb-Pb data takingand they were provided with only a few tens of collidingbunch pairs. Around LHCb, the optics was similar tothat of the preceding p-p run, with β ∗ = 3 m. In 2018,the β -function in LHCb was reduced to β ∗ = 1 . S b = 40 λ RF (cid:39) c ns, to S b = 30 λ RF (cid:39) c ns, in termsof the RF wavelength λ RF (cid:39) . c ns. In consequence,LHCb naturally received about 10 times more collisions .Even though higher peak luminosities would have been At design luminosity of L = 1 × cm − s − , P BFPP = 26 W. The IP of LHCb is displaced by 15 λ RF (cid:39) . c ns with respectto the symmetry point at s = 7 C/
8, where the LHC circumfer-ence C = 35640 λ RF . In “natural” filling schemes of the LHC, byquadrant, each bunch is located at s = jC/ mS b , for 0 ≤ j < ≤ m < C/ (4 S b ), behind the leading bunch. Then encountersoccur at s = kC/ nS b / ≤ k <
8, 0 ≤ n < C/ (8 S b ),which always includes the locations of ATLAS, ALICE andCMS at s = 0 , C/ , C/ not necessarily that of LHCb at s = 1039 C/ S b = 10 λ RF (cid:39)
25 ns (usedfor p-p) and S b = 30 λ RF (cid:39)
75 ns naturally provide a large num-ber of collisions to LHCb. For the previous Pb beam spacing of S b = 40 λ RF (cid:39)
100 ns, or the future S b = 20 λ RF (cid:39)
50 ns, some possible, beams were levelled at L = 1 × cm − s − (as in ALICE) to reduce the risk of quenches (see below). B. BFPP Orbit Bumps
The importance and consequences of the secondarybeams were only realised in their entirety after the fi-nal layout of the LHC had been defined (around the year2000) and no potential counter-measures could be im-plemented into the cold sections of the accelerator lat-tice [1] before the start-up. Early calculations estimatedthat BFPP losses would be able to quench magnets al-ready below the nominal luminosity [13] and mitigationmeasures using the deflection of the secondary beams bymeans of orbit bumps were investigated [15] well beforethe first heavy ions circulated in the LHC.
1. The Technique
The dispersion suppressor (DS) is the lattice sectionthat directly connects the regular FODO cells in the arcsto the straight sections on either side of each IP. EachDS accommodates four superconducting quadrupoles andeight superconducting dipoles arranged in four cells,numbered 8-11 (see illustration of beam-line elements inFig. 1 or 2). The last cell (11) is longer than the previousthree and contains an extra drift space of approximatelythe length of a dipole magnet (“missing dipole”). Thisspace contains a connection cryostat (labelled LEGR orLEFL to the right or left of the IP) that bridges thevacuum, electrical and cryogenic systems to the first op-tically periodic arc cell.Thanks to the providential combination of lattice ar-rangement and optics around IP1/5, the impact locationof the BFPP beam (see Figs. 2 and 3) naturally lies inthe DS at the end of the second superconducting dipoleof the 11 th cell (corresponds to the 8 th dipole in the DS,labelled MB.B11) downstream the IP. This is the dipolethat is followed by the empty connection cryostat with-out magnet coil.This situation allows the use of a horizontal orbit bumparound the impact location that pulls the secondarybeam away from the aperture just enough to move thebeam losses out of the dipole and into the connectioncryostat . This is beneficial since the superconductingbus bars in the connection cryostats, which connect theDS and arc magnets in series, have a much higher steady-state quench level than the magnet coils themselves. A injected bunch trains have to be displaced from the natural po-sitions to obtain any collisions at all in LHCb. This generallydeprives the other experiments of some collisions. Note that this orbit bump will also deflect the main beam to-wards the opposite side of the beam pipe by a maximum of thebump amplitude, usually around 3 mm, without compromisingmachine protection. rough estimate lies around 200 −
300 mW / cm [30, 31]at 7 Z TeV instead of tens of mW / cm for the magnets.Although this estimate is very rough, the power deposi-tion is much lower (see Section III D) and therefore therisk of quenching is low. In addition, the bus bars arelocated further away from the vacuum chamber and aretherefore less exposed to the shower initiated by the im-pacting ions.Figure 2 shows the main and BFPP beam trajectorieson both sides of IP5 (Beam 1 travels to the right, Beam 2to the left). Here the natural trajectories are comparedto the ones modified by a three-magnet orbit bump withan amplitude of −
2. Operational Experience with the Bumps
The bump amplitudes that were used operationally in2015/18 are listed in Table II. The bumps were used forthe first time in 2015 but, based on the quench limitestimates at that time, it was thought that they were notyet strictly necessary for the expected luminosity reacharound L = 3 × cm − s − . Therefore, in that yearonly, the computed bump amplitudes were taken at facevalue. The optimization procedure described above wascarried out very briefly, mainly to get experience with thenew technique. In places where the observed loss patternwith the calculated bump amplitude indicated that theBFPP beams were not well placed in the cryostat, a smallvariation of bump amplitude was tried, but the calculatedvalues were retained for physics operation.Since it is important for Section III it should be notedhere that already during the setup of the bumps a left-right asymmetry was observed in IP5. While the calcu-lated bump moved the losses well into the cryostat on theright (outgoing Beam 1), high losses were still observedin the dipole on the left (outgoing Beam 2) with a simi-lar bump amplitude. The main origin of this effect waslater identified to be a misalignment of the real aperturecompared to the theoretical one, as will be discussed indetail in Section III.With the experience gained from the 2015 luminosityoperation and quench experiment, more care was takento set up the bumps during the commissioning in 2018.Detailed bump scans were executed on both sides of IP1,IP2 and IP5 and the empirically optimized amplitudeswere implemented into the cycle. Table II shows thatin IR1 the optimal bumps were found to be symmet-ric, while in IR5 a large left-right asymmetry was stillpresent. However, contrary to 2015, a smaller bump wasrequired on the left side of IP5 in order to obtain a losspattern similar to that on the right side. The reason forthe smaller bump in 2018 is the sum of two effects.1. Alignment measurements of the DS elementsaround cell 11 in 2020 indicated a collective hor-izontal shift of the beam-line elements towards theoutside of the ring of the order of a few hundredmicrometers. Depending on the real significance ofthis alignment change, it could lead to an impactposition slightly further downstream.2. Nevertheless, the main influence comes from thedifferent IP optics with a smaller β ∗ and largercrossing-angle in 2018. This led to a variation ofthe local dispersion just behind the IP and a differ-ent deflection of the BFPP particles, shifting theirimpact location further downstream (see Fig. 4).On the left of IP5, the difference from 2015 is of theorder of a few meters, while on the right the effect is TABLE II. Operationally used BFPP bump amplitudes inmillimeters.
IP1 -3.2 -2.75 -2.6 -2.6IP2 -3.0 -3.0 -2.6 -2.0IP5 -3.0 -2.6 -1.6 -2.5IP8 0.0 0.0 0.0 0.0 much reduced. The left-right asymmetry arises be-cause the lattice and optics symmetry between theoutgoing Beam 2 on the left and outgoing Beam 1on the right is not perfect. There are small dif-ferences in the matching quadrupole strength andlocations between each side of the IP.Already in 2015, it was estimated that the levelledluminosity of L = 1 × cm − s − in IP2 was proba-bly too low to quench a dipole with the BFPP beams.However, in the absence of experimental confirmation atthe time, the bumps were nevertheless designed and im-plemented in IR2. As will be further detailed in Sec-tion IV B, the bumps in IP2 could only distribute thefull load of losses over two cells, rather than move theminto the connection cryostat. This turned out to be ad-equate in Run 2, but will not be enough to mitigate thequench risk for the much higher luminosity that will beprovided to ALICE after its upgrade (see Section III).No bumps have been implemented in IR8. For the lownumber of collisions in 2015, losses naturally stayed be-low the quench limit. With the 75 ns bunch spacing inthe second half of the 2018 run, the potential luminos-ity in IP8 became comparable to the other experiments.Since the local geometry and impact distribution are dif-ferent from IR1/5, it cannot be directly assumed thatthe same power deposition and luminosity limit experi-mentally found in 2015 for IR5 (see Section III) appliesalso to IR8. Therefore, LHCb was conservatively levelledat the same value as ALICE ( L = 1 × cm − s − ) toprotect from quenches as well as share luminosity.The bumps have proven to be very efficient and allowat least peak luminosities up to L = 6 × cm − s − (measured in 2018) in IP1/5. So far no luminosity pro-duction fill has been interrupted by a quench or abort(beam dump) due to BFPP losses. C. Machine Protection Aspects
In order to ensure the machine safety when operatingwith these special orbit bumps a number of measures areapplied. Because of the different loss mechanisms andbeam optics in heavy-ion operation, as compared to thepreceding p-p operation, the collimation system has tobe re-validated and BLM abort thresholds have to beadjusted. The abort thresholds are typically set so thatthey trigger the extraction of the beams before beam-
440 435 430 425 420 415 410s [m] from IP50.0160.0180.0200.0220.024 x [ m ] MB.A8L5.B2MB.B8L5.B2MB.A9L5.B2MB.B9L5.B2MB.A10L5.B2MB.B10L5.B2MB.A11L5.B2MB.B11L5.B2LEFL.11L5.B2MB.A12L5.B2MB.B12L5.B2MB.C12L5.B2 BFPP1 2015, -3mmBFPP1 2018, -3mm B L M s i g n a l [ G y / s ] Beam 2
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410 415 420 425 430s [m] from IP50.0160.0180.0200.0220.024 x [ m ] MB.A8R5.B1 MB.B8R5.B1 MB.A9R5.B1 MB.B9R5.B1 MB.A10R5.B1 MB.B10R5.B1 MB.A11R5.B1 MB.B11R5.B1 LEGR.11R5.B1 MB.A12R5.B1 MB.B12R5.B1 MB.C12R5.B1BFPP1 2015, -2.6mmBFPP1 2018, -2.6mm B L M s i g n a l [ G y / s ] Beam 1
BLM 2015, -2.6mmBLM 2018, -2.6mm
FIG. 4. 2015 (black) and 2018 (purple) MAD-X BFPP trajectories (straight lines) and BLM loss signals (correspondingcolors) in cell 11 left and right of IP5 for operational bump amplitudes in 2015 (on the left for − − . induced quenches can develop.The BLM thresholds have to be adapted before ionbunches are put in the machine for the first time in arun. In particular, dedicated abort thresholds, derivedwith FLUKA shower simulations, are implemented forBLMs at BFPP loss locations next to the four experimen-tal insertions. Threshold adjustments are also needed inthe betatron cleaning insertion and the neighbouring dis-persion suppressor to account for the reduced cleaningefficiency compared to protons. The thresholds are thenempirically fine-tuned during the initial run period basedon the measured BLM response.As a standard procedure when commissioning a newbeam mode or optics in the LHC the collimation hier-archy is re-validated by artificially exciting a low inten-sity beam to provoke losses that are observed all aroundthe circumference by the BLM system. This provides aso-called loss map that is used to verify the collimationcleaning efficiency and that the highest loss rates remainconfined to the primary collimator locations.Since 2015, a special set of loss maps and as well opticsmeasurements have been performed with the maximumpossible amplitude of the BFPP bumps that might everbe deployed during operation to ensure that they gener-ate no unexpected aperture bottlenecks or optics distor-tions. Smaller bump amplitudes are considered to be safeif the largest ones are. Therefore, the later optimisationof the BFPP bump amplitude, which requires a sufficientluminosity signal, is allowed to set the operational am-plitudes to a smaller value. III. BFPP QUENCH TEST
In order to probe the luminosity limit for Pb-Pb colli-sions and to better predict future performances, a dedi-cated test was performed in 2015 that used the BFPP1beam to provoke a controlled beam-induced quench of a bending dipole. The goal of the test was to experimen-tally determine the dipole quench level for steady-statelosses at 6 . Z TeV. Other controlled quench experi-ments, based on different kinds of loss techniques, hadbeen previously performed at 3 . Z TeV and 4 Z TeV inRun 1 [35], but some uncertainty remained concerningthe expected quench level at higher energies. For steady-state losses at 7 Z TeV, i.e. at the LHC design energy, theminimum quench power density for main bending dipoleswas estimated to be 22 −
46 mW / cm according to elec-trothermal models and cable stack measurements [35] (asin Ref. [35], the power density is given as an average den-sity across the cable’s cross section). At 6 . ∼ −
55 mW / cm , while atthe Pb operation energy of 6 . Z TeV, the quench levelswere expected to be a few mW/cm higher. At the out-set of the BFPP quench experiment, it was not clear ifthe peak luminosity which could be achieved in 2015 wassufficient to induce a quench.The BFPP beams can provide a very clean loss sce-nario compared with other mechanisms that might in-duce quenches in proton or heavy-ion operation of theLHC. The power deposition in the magnet coils can bereconstructed with FLUKA particle shower simulationswhich can then be used to benchmark electrothermalmodels. Using the BFPP1 beam to induce a quench hasthe advantage that the impact point in the magnet canbe controlled by modifying the orbit bumps. In this way,quenches at the end of the magnet, where the complexcoil geometry makes it more difficult to reconstruct thepower density, can be avoided. Furthermore, the powerof the BFPP beam is directly dependent on the luminos-ity, which can be controlled with the beam separationat the IP. Preliminary results of this quench experimentwere already presented in Ref. [22]. TABLE III. Average beam parameters in the BFPP quenchexperiment. Errors indicate the standard deviation. Emit-tances ε n ( x,y ) are normalised values.Fill number 4707Ions per bunch N b (1 . ± . × Bunches colliding in IP5 k c σ z . ± . ε n ( x,y ) (Beam 1) (2 . , . ± . µ m ε n ( x,y ) (Beam 2) (2 . , . ± . µ m A. Setup of the Experiment
The experiment was performed on 8 December 2015,with the highest intensity and lowest transverse emit-tances available at that date to maximise the likelihoodof a quench. The beams were prepared as for a stan-dard physics fill up to the point of being put in collision.The average beam parameters at that time, just beforestarting the experiment, are listed in Table III.The loss location left of IP5 was chosen as most propi-tious for the experiment as it exhibited the highest BLMsignals in the preceding fills and the beam impact pointlay further inside the dipole in the absence of the orbitbump. On the right of IP5 and on both sides of IP1, thebeams would impact closer to the end of the dipole or inthe interconnect if no orbit bump was applied.Before conducting the experiment, the BLM abortthresholds around the impact location in cell 11 left ofIP5 were raised to avoid premature beam dumps. Fur-ther details of the procedure are given in [36].
B. Conducting the Experiment
Once the beams were colliding, with the BFPP bumpsin place as in normal operation, they were re-separated inall IPs in order to reduce burn-off and save peak luminos-ity for the experiment. The evolution of the luminositymeasured by CMS (black line) can be followed through-out the experiment in Fig. 5 together with some BLMsignals around the BFPP impact location to the left andright of IP5. The vertical separation at IP5 was reducedsufficiently to discern a pattern on the BLM signals thatcould later point clearly to the impact point of the BFPPbeam in the bending magnet based on a comparison withFLUKA simulations.From here the BFPP orbit bump left of IP5 was re-duced from − . − . . µ m, waiting a few minutes at eachstep for conditions to stabilize (red highlighted periodin Fig. 5). After performing the 4th step and arrivingat the head on position, a quench of MB.B11L5 devel-oped after around 20 s at an instantaneous luminosity of L ≈ . × cm − s − in CMS. C. BFPP Impact Location and Distribution
The exact impact point and distribution dependsstrongly on the real beam screen aperture and alignment.Even a small deviation of the real aperture from the theo-retical one will lead to significant differences between ob-servation and simulation of the particle shower, and thusthe obtained power deposition and quench limit. As anexample, the ideal mechanical design value of the beamscreen aperture is 23 .
15 mm, while the nominal apertureused in the standard LHC MAD-X [32] files is reduced to22 . x and y off-sets of the beam screen within each magnet were mea-sured with a longitudinal resolution of 10 cm before theirinstallation in the tunnel. In order to obtain the realaperture, this offset data has to be superimposed on theideal beam screen alignment (23 .
15 mm). The differencebetween the nominal and this corrected aperture modelis indicated in Fig. 6 for the region around the BFPPimpact location.Looking at the difference between the grey (nominalMAD-X aperture) and green (real aperture from mea-surement) shaded areas reveals that the real aperture canfeature obstacles, presenting surfaces that are not paral-lel to the beam direction to a significant degree. Whilethese may be neglected for other purposes they turnedout to be significant for the results of the quench testdiscussed here. On the right, the measured aperture iswithin the tolerances of the nominal value. On the left,however, the tolerance value is exceeded by two spikes,where the one around s = −
417 m influences the BFPPloss location and distribution for orbit bump amplitudesrelevant for the quench test.The effect of this aperture deformation on the impactdistributions in s -direction on the beam screen is shown B L M [ m G y / s ] Inversion BFPPbump left of IP5 Stepwiseluminosityincrease
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07 21 :
08 21 :
09 21 : Time B u m p [ c m s ] BLMAI.11R5.B1E24-MBBBLMBI.11R5.B0T20-MBB-LEGR-11R5BLMEI.11R5.B1E10-LEGR BLMEI.11R5.B1E20-LEGRBLMEI.11R5.B1E30-LEGRCMS:LUMI-TOT-INST
FIG. 5. Evolution of BLM signals around the BFPP impact point to the left (Beam 2) and right (Beam 1) of IP5, whilestepwise inverting the orbit bump from − .
430 425 420 415 410 405 400s [m] from IR5 (left)0.0210.0220.0230.0240.0250.0260.027 x [ m ] MB.B11L5.B2LEFL.11L5.B2 Beam 2
Left of IR5 nominal aperturereal aperture+0.5 mm 0.0 mm-1.0 mm-2.0 mm -3.0 mm-4.0 mm-5.0 mm
400 405 410 415 420 425 430s [m] from IR5 (right)0.0210.0220.0230.0240.0250.0260.027 x [ m ] MB.B11R5.B1 LEGR.11R5.B1Beam 1
Right of IR5 nominal aperturereal aperture+0.5 mm 0.0 mm-1.0 mm-2.0 mm -3.0 mm-4.0 mm-5.0 mm
FIG. 6. Standard (22 mm, grey shaded area) and real (green shaded area) horizontal aperture from measurement, superimposedwith the BFPP beam trajectory around the impact point left and right of IP5 shown for orbit bump amplitudes between +0 . −
430 425 420 415 410s [m] from IP5020004000600080001000012000 h i s t o g r a m c o un t s MB.B11L5.B2LEFL.11L5.B2
Beam 2 0.0100.0150.0200.0250.030 x [ m ] real aperture+0.5 mm -1.0 mm-3.0 mm -5.0 mm FIG. 7. BFPP beam distribution as lost in s -direction onthe beam screen, assuming the measured aperture along theimpact location to the left of IP5 and the 2015 operational op-tics configuration with different BFPP bump amplitudes. Thetransparent rectangles on the bottom indicate the positionsof the dipole MB.B11L5 and connection cryostat LEFL.11L5. in Fig. 7. As the orbit bump is increased (BFPP tra-jectories for different bump settings are superimposed),the impact location changes less for a given change inbump amplitude and thus occurs more upstream thanexpected. Further, the shape of the distribution changessignificantly with respect to the nominal case (which canbe assumed to be approximately Gaussian). This is espe-cially evident around the − D. Analysis with FLUKA
Particle shower simulations were carried out withFLUKA to evaluate the peak power density deposited inthe magnet coils during the quench, providing, in turn,a tentative estimate of the steady-state quench level ofmagnets at 6 . Z TeV. FLUKA has been benchmarkedpreviously against LHC BLM measurements for differentkinds of beam losses [37]; the benchmarks showed thatthe simulations can reproduce measured signals within afew tens of percent in case of well known loss conditions.The studies presented here were based on a similar sim-ulation setup, using a realistic geometry model of themagnet, including beam screen, cold bore, coils, collarsand yoke.To verify the predictive power of the simulation modelfor the BFPP experiment, simulated BLM signals werecompared to measurements. The particle shower simu-lations were based on BFPP1 loss distributions trackedwith MAD-X [32], assuming an orbit bump of +0 . . . z ∆ r ∆ φ = (10 cm) × (0 . × (2 ◦ ). Figure 10 presentsthe longitudinal distribution of the peak power density inthe coils obtained with the real and ideal FLUKA aper-ture models, respectively. Both, the peak power densityat the inner edge of the cable and the radially averageddensity over the cable width are shown. As the heat hasenough time to spread across the cables’ cross-section,one typically uses the radially averaged power density toquantify the quench level for steady-state losses. Themaximum radially averaged power density is estimatedto be around 20 mW / cm in presence of aperture im-perfections, while it is around 15 mW / cm if an idealbeam screen surface is assumed. This remarkable differ-ence in the results is much more dramatic than for theBLM signals shown in Fig. 9 and can be explained bythe proximity of the coils to the beam screen. Becauseof this proximity, the power density distribution in thecoils depends on detailed features of the loss distribu-tion. These features cannot be resolved by the BLMssince they are located outside of the cryostats and aretherefore exposed to the far shower tails leaking throughthe massive magnets. The showers smear out these de-tailed characteristics of the loss distribution.Apart from the aperture misalignment, the real lossdistribution and hence the maximum power density inthe coils depends on the crossing angle, the horizon-tal and vertical emittance, the momentum spread, im-perfections such as small deviations from nominal mag-netic field strengths or local inhomogeneities of the beamscreen surface at the impact location, but also possiblevariations of beam and optics parameters. Consideringthese uncertainties, it is estimated that the error on thecomputed peak power density is at least a few tens ofpercent, possibly up to a factor of two.Finally, this could also hint at a non-negligible influ-ence of aperture imperfections in magnet quenches fromlocalized losses. If, as our model indicates, BLM sig-nal patterns do not vary widely with aperture imperfec-tions but peak power density deposition does, a greateramount of power density than initially expected could bedeposited in the magnet coils due to a local aperture im-perfection. This could not be discerned from a real timemonitoring of BLM signals during operation, potentiallyhiding the risk of a magnet quench. Beam abort thresh-olds must therefore incorporate a safety margin. E. Conclusion from the Experiment
The reconstructed peak power density in the dipolecoils during the quench (20 mW / cm when includingaperture imperfections) is a factor of two lower than thelower bound of the aforementioned steady-state quench0 inst =2.3 x 10 cm -2 s -1 B L M s i gn a l ( m G y / s ) Distance from IP5 (m)Experimental LHCFLUKA -414.3mFLUKA -414.8mLEFL.11L5 MB.B11L5
FIG. 8. BLM signal comparison between experimental datafrom the quench test (blue) and FLUKA data assuming twodifferent loss locations (red and green). The particle distribu-tions for the different loss locations assumed in the simulationsare shown in histograms of the corresponding color. -426 -424 -422 -420 -418 -416 -414 -412 -410L inst =2.3 x 10 cm -2 s -1 B L M s i gn a l ( m G y / s ) Distance from IP5 (m)Experimental LHCFLUKA real apertureFLUKA ideal apertureLEFL.11L5 MB.B11L5
FIG. 9. BLM signal comparison between experimental datafrom the quench test (blue) and FLUKA data assuming botha real (orange) and an ideal (dark green) aperture model.Note that the blue and dark green lines are identical to theones in Fig. 8. P ea k po w e r d e n s it y M B c o il s ( m W / c m ) Distance from IP5 (m)Inner edge cable real apertureInner edge cable ideal apertureRadial average cable real apertureRadial average cable ideal aperture
FIG. 10. Peak power density in the MB.B11L5 coils duringthe quench test estimated by FLUKA simulations assumingboth an ideal and a real aperture model. level predicted by electrothermal models (40 mW / cm at 6 . µ mhigh. Although the loss scenario was different and cannotbe compared to BFPP losses, a discontinuity could havelikewise increased the peak power density in the presenttest. Uncertainties also remain concerning the aperturealignment, which had to be reconstructed from severaldifferent data sets measured over several years. The im-pact distribution further depends on the instantaneousbeam parameters and optics properties. It is difficult toexactly quantify these sources of error.It is also worth noting that during 2015 physics oper-ation it was possible to reach L = 3–3.5 × cm − s − with an inefficient bump, while a quench occurred at L = 2 . × cm − s − during the experiment. We re-call that the loss pattern at the left side of IP5, wherethe experiment was performed, suggested that the oper-ational orbit bump did not fully move the BFPP beaminto the connection cryostat and that the main fractionstill impacted in the end part of the dipole.Because of the sensitivities mentioned, the found peakluminosity leading to the quench cannot directly be as-sumed to be equivalent for other IPs or under differentimpact conditions, e.g., new optics or a new impact po-sition within the magnet. The results from the quenchtest were nevertheless used in 2016 and 2018 to adjustthe BLM thresholds for heavy-ion operation in all IPssince the test had shown that quenches were possible atlower luminosities than assumed in the original thresholdsettings.To reduce the remaining uncertainty of the steady-state quench level of dipoles, a second quench test in adifferent location was scheduled in the last few hours ofthe 2018 run. Unfortunately it could not be carried outdue to an unexpected failure which meant that beamswere not available from the injectors. The experimentremains to be repeated in the next Pb-Pb run, currentlyforeseen at the end of 2022. IV. FUTURE HEAVY-ION RUNS
The constant performance improvement in the injec-tors and LHC since the first heavy-ion collisions pro-vided the opportunity to briefly operate very close to the1HL-LHC target luminosity [38] already in 2018. Nev-ertheless, the 2015 quench test confirmed earlier calcula-tions [12, 15] that BFPP ion losses would limit luminositybelow the HL-LHC target of L = 7 × cm − s − , al-though the limit was found to be higher than originallyestimated by these studies. Various options have beenconsidered to reduce the power deposition in the super-conducting magnets. The following section details themitigation measures planned in all IPs. Hardware up-grades are only foreseen in IP2 and are being installedin the second long shutdown (2019–2021) to be ready forLHC Run 3, when the heavy-ion program enters in its“high-luminosity” era. A. Mitigation Strategy IP1/5: Orbit Bumps
Without the possibility to install special hardware forthe alleviation of BFPP beam losses, mitigation measuresusing the deflection of the secondary particles by meansof orbit bumps were investigated well before the first ionrun of the LHC [14, 15]. The final orbit bump technique,explained in Section II B, was applied already very suc-cessfully in LHC Run 2. With the possibility to movethe impact location of the BFPP ions into the connectioncryostat, a peak luminosity over L = 6 × cm − s − was reached in IP1/5 without quenching superconductingmagnets.Nevertheless, by moving these losses further down-stream, potential risks to other magnets and to the bus-bars in the connection cryostat need to be carefully as-sessed, in order to avoid new limitations for the fu-ture luminosity performance foreseen in the HL-LHCera. While the losses in the MB.B11 almost completelydisappear with the BFPP bump in place, the adjacentquadrupole MQ.11 (blue rectangle in Fig. 3) is exposedto a higher power deposition. Simulation studies per-formed in Ref. [23] confirm that, even for HL-LHC spec-ifications of a luminosity of L = 7 × cm − s − andat a beam energy of 7 Z TeV, the peak power density innearby superconductors, i.e., dipole MB.B11, bus barsof the connection cryostat and the adjacent quadrupoleMQ.11, would safely remain below the quench level withmore than a factor 10 margin when operating with BFPPbumps. Nevertheless, the operational margins for thecryogenic system and radiation effects on electronics re-main to be evaluated.Apart from the power deposition in the magnet coils,the dynamic heat load to be evacuated by the cryogenicsystem could become a limitation. This was also stud-ied in Ref. [23]. According to Eq. (1) the BFPP beamscarry about 180 W under HL-LHC conditions, which islost in nearby accelerator elements. Ref. [23] estimatesthat around 75% of that power goes into the cold masswhen the ions are lost deep inside the dipole. In the DSregion, it is potentially possible to extract 150 W (120 Wdynamic plus static loads) from cold mass elements at1 . x [ m ] M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B M B . A R . B M B . B R . B L E C L . R . B M B . A R . B M B . B R . B M B . C R . B Beam 1 Pb (main beam) Pb (with bump) Pb (BFPP1) Pb (with bump) FIG. 11. Main (12 σ ) and BFPP1 (1 σ ) radial beam envelopesand aperture (grey) in Beam 1 direction right of IP2 (at s = 0). Beam-line elements are indicated schematically asrectangles. Dipoles in light blue, quadrupoles in dark blue(focusing) and red (defocusing). The BFPP1 beam impactsin the aperture of the second superconducting dipole magnetof cell 10. An orbit bump around MQML.10R2 of − s = 430 m) placed in the empty connection cryostatin cell 11. dundancy is jeopardized. With the orbit bumps in place,more than half of the beam power goes to the connec-tion cryostat, with a large fraction of it (around 45 W)being deposited in lead shielding plates around the vac-uum chamber that are thermalized to 50 −
65 K. Shiftingthe losses from the upstream dipole into the connectioncryostat therefore also has a positive effect on the overallload to the cryogenic system.This, in combination with the operational experiencegained at record luminosity close to the HL-LHC targetin 2018, confirms that the orbit bumps a robust solutionto guarantee the accessible luminosity for ATLAS andCMS. Therefore, this technique was confirmed as the HL-LHC baseline strategy for BFPP quench mitigation inthese IPs.
B. Mitigation Strategy IP2: DS Collimators
Because of the different optics in IR2, that featuresthe opposite quadrupole polarity, the secondary beamscannot be deflected into the connection cryostat by us-ing an orbit bump. Here the secondary beams are lostalready in cell 10 in MB.B10 (6 th dipole in DS). Sincethe periodicity of the dispersion function is shifted onecell downstream, the locally generated dispersion is largeenough to move the BFPP ions out of the ring acceptancein its first peak (see Fig. 11).An orbit bump with its maximum around Q10 couldonly be used to move some (or all) of the losses to cell 122(MB.C12) . This did not impose limits during the 2015and 2018 runs because of the levelled luminosity in IP2,which kept the BFPP beam power below the quenchlimit.In order to reach the peak luminosity in IP2 foreseen inHL-LHC, similar to values in IP1/5 when operating withorbit bumps, the installation of an additional collimatorin the dispersion suppressor, a so-called TCLD , that canabsorb the impacting BFPP ions, is necessary. However,due to the compact lattice design of the LHC, no space isavailable to additionally install such a collimator in thelattice without removing existing equipment. Two op-tions have been proposed in the scope of the collimationupgrade programme for HL-LHC [38–40].The first option, which was considered in the past butis now obsolete, is to remove and replace an existing stan-dard main dipole magnet (8 T) with a pair of shorter,higher field (11 T) dipoles, which would create space fora tungsten collimator between them. In fact this solu-tion was chosen as baseline for installing TCLDs in theDS adjacent to the betatron collimation region in IR7,where the collimators are needed to avoid quenches in-duced by fragments leaking out of the collimation inser-tion [38, 41–44]. In IR2, this assembly would have to beinstalled upstream of the MB.B10 on both sides of the IPin order to intercept the BFPP ions before they impactin cell 10.The second, adopted, solution does not require newmagnets. If an orbit bump is used to pull the BFPPparticles out of the aperture in cell 10, they can con-tinue to travel downstream until they are absorbed by acollimator installed in the connection cryostat in cell 11.Ref. [23] estimates the power deposition in the surround-ing superconductors to be uncritical and states that theabsorbing properties of the tungsten jaws also reducesthe heat load to be evacuated by the cryogenic system.The installation of such a devices on the outgoing beamson both sides of IP2 is foreseen during the current sec-ond long shutdown (LS2) and will enable the HL-LHCperformance reach in Pb-Pb collisions for ALICE fromRun 3. C. Mitigation Strategy IP8: Luminosity Levelling
Since 2018, LHCb has requested luminosities compa-rable to the other experiments bringing it into a regimewhere BFPP losses become a concern. The dispersion has a minimum (not a maximum as in IP1/5)around the connection cryostat in cell 11 such that the calculatedtrajectory (without impact) of the BFPP1 beam is closer to thecentral main beam orbit in cell 11 compared to cell 10 and 12. Naming follows the LHC naming convention. As this device isa collimator in the DS that is responsible for absorbing lumi-nosity products, the letter code is the following: TC = TargetCollimator, L = Luminosity, D = Dispersion Suppressor.
Similarly to IR2, the optics in IR8 do not allow theBFPP ions to be moved into the connection cyrostat bymeans of an orbit bump. Even an IR2-like bump that dis-tributes losses over two cells seems inefficient. No TCLDinstallation or other upgrade options are presently fore-seen. This leaves luminosity levelling to a target safelybelow the quench limit as the only option for BFPPquench mitigation in IR8.
D. Operation with Lighter Nuclei
LHC operation with lighter nuclear species has beendiscussed for many years but has so far not been in-cluded in the official planning. The great success, andhigh scientific output, of the very short xenon-xenon runin 2017 [45] increased the interest in collisions with lighterions in the experimental community [46, 47].From the point of view of collider performance, es-pecially with respect to secondary beams, the operationwith lighter ions would be beneficial. Event cross-sectionsfor BFPP ( σ BFPP ) and other ultra-peripheral interac-tions strongly depend on high powers of the particle’scharge number ( Z ) [16]: σ BFPP ∝ Z (2)Since the change in magnetic rigidity in the ultrape-ripheral processes are significantly different, the consid-erations relating to impact locations, orbit bumps andcollimators discussed above for Pb ions do not carry overdirectly. However, as illustrated by Eq. (1), the powercarried by the secondary beams in collisions of lighterions drastically decreases with respect to Pb-Pb colli-sions, naturally reducing the power deposition in the su-perconductors of the DS. Furthermore, the smaller cross-sections lead to a reduced burn-off rate from such in-teractions, which results in a longer luminosity lifetime,leaving more ions for hadronic interactions. Approximateevaluations of these effects are given in [46]. V. CONCLUSIONS
In heavy-ion operation of the LHC, secondary beamscreated by bound-free pair production processes in thecollision dissipate a significant power in the dispersionsuppressor regions around the IPs. In the absence ofmitigation measures, they would quench various super-conducting magnets and limit present and future energiesand luminosities to values below those already demon-strated.The BFPP secondary beams were used to measurethe steady-state quench level of the LHC dipole mag-nets at 6 . Z TeV in an experiment performed in De-cember 2015. A quench was observed at a luminosityof L ≈ . × cm − s − . The corresponding peakpower density in the magnet coils was estimated to about20 mW / cm .3Since the 2015 Pb-Pb run, orbit bumps at BFPP losslocations have been routinely used and successfully elim-inate the quench risk from the BFPP secondary beams inIP1/5. In the 2018 run, record peak luminosities of morethan L = 6 × cm − s − were reached and no physicsfill was interrupted by a quench or pre-emptive abort.This demonstrated the robustness of the orbit bump tech-nique and its feasibility for the use in IP1 and IP5 underHL-LHC specifications. The installation of new collima-tors around IP2 will allow the future HL-LHC Pb-Pbtarget luminosity to be provided to the upgraded ALICEexperiment. FLUKA simulations, benchmarked amongothers with the presented quench test, demonstrate thatthe proposed alleviation techniques are efficient and pro-vide a safety factor of at least 10 beyond the HL-LHCPb-Pb luminosity reach. These studies underline that,in all three IPs, the dissipation of losses into the connec-tion cryostat provides an even distribution of heat loadamong the various components, facilitating its evacuationby the cryogenic system. Nevertheless, the evaluation ofthe safety margin with respect to other limitations likecryogenic load and radiation effects on electronics need some further study.The luminosity reach for the LHCb experiment in IP8strongly depends on the bunch spacing and sharing oftotal available luminosity with the other experiments.As no accelerator upgrades are foreseen in IR8 and or-bit bumps are ineffective, luminosity levelling to a targetsafely below the quench limit remains for now the onlyoption for BFPP quench mitigation here. ACKNOWLEDGMENTS
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